Many of them have legitimate theraputic use and aren't illegal if taken under prescription, or for legitimate medical reasons. HGH, for example, was often used to rebuild muscle strength during rehabilitation from surgery or injury.

Bear in mind that cortico-steroid has legitimate medical uses, and after Armstrong tested positive for it in the 1999 TdF, he produced a very convenient prescription for it to treat his "saddle sores". Many have felt since day 1 that it was back-dated.